Recent environmental regulations propose more stringent containment and/or treatment regulations for bleach plant effluent containing adsorbable organic halides (AOX), biologically recalcitrant chemical oxygen demanding (COD) materials, toxicity and color containing compounds. While these more stringent regulations may be met with currently available treatment systems, the costs for achieving the proposed limits are excessive in many instances. In some situations major plant modifications may be required in order to effectively reduce the subject pollutants to the required level. In other situations, converting from elemental chlorine-free bleaching (ECF) to totally free chlorine bleaching (TCF) may be the most cost effective means to achieve the reduction in pollutants proposed in the environmental regulations. However, the conversion of bleaching plants from ECF to TCF may require major plant modifications.
Conventional pulp bleaching plants use halogen agents, which are the major source of AOX in the effluent streams. A conventional bleaching sequence for softwood pulp treated in accordance with the sulfate process is EQU (C+D)E.sub.1 DE.sub.2 D
wherein (C+D) is a stage for the addition of chlorine (C) and chlorine dioxide (D), either simultaneously or sequentially; D is a chlorine dioxide addition stage, and E.sub.1 and E.sub.2 are alkaline extraction stages, optionally with addition of peroxide (E.sub.p) and/or oxygen (E.sub.op or E.sub.o). In the above bleaching sequence, the (C+D) stage and the E.sub.1 stage are often referred to as the prebleaching stages. The sequence DE.sub.2 D is called the final bleaching stage. In an elemental chlorine-free pulp bleaching plant, a bleaching sequence such as D.sub.o E.sub.op D may be used.
The reaction products formed in the bleaching stages using halogen-containing compounds give rise to discharges containing halogenated organic compounds. These compounds are measured as absorbable organic halogen (AOX). When chlorine dioxide is used instead of elemental chlorine, the AOX may be significantly reduced. Processes using only chlorine dioxide in the prebleaching stage are typically known as elemental chlorine-free (ECF) bleaching processes. While the use of chlorine dioxide in place of elemental chlorine has reduced the level of AOX in plant effluent, there continues to be a need to further reduce the level of these compounds.
In addition to AOX, pulp bleach plant effluents typically have a high chemical oxygen demand (COD) and a high color content. Conventional primary treatment systems are designed to reduce only suspended solids (SS), not AOX, COD, and color. Other treatment systems may reduce the AOX and color of the effluent but fail to reduce the COD. Secondary or biological treatment systems are useful for reducing the biochemical oxygen demand (BOD) of the effluent but typically do not reduce color and are only moderately effective in removing AOX and COD.
Accordingly, it is an object of the present invention to provide a cost effective method for reducing pollutants in the effluent discharged from a pulp bleaching plant.
Another object of the invention is to provide a method for treating filtrate from a pulp bleaching plant whereby the effectiveness of secondary and/or tertiary treatment is increased.
Still another object of the invention is to reduce the amount of pollutants in plant filtrate streams without adversely affecting the biological treatment systems used for subsequent treatment of the filtrate streams to reduce BOD.
Yet another object of the invention is to condition filtrate streams so that subsequent biological treatment becomes more effective.
An additional object of the invention is to provide a method for treating pulp bleach plant effluent which reduces the AOX, COD and color of the effluent.
A further object of the invention is to provide a method for treating pulp bleach plant effluent which enables reduction of pollutants in the plant discharge stream to acceptably low levels in accordance with applicable standards.
A still further object of the invention is to provide a method for treating pulp bleach plant effluent which avoids radical or expensive modifications in existing plant equipment or processes.